The invention relates to a picture display device comprising a colour display tube having an elongate display screen with a short axis and a long axis, which screen is provided on an inner surface, and a shadow mask arranged in front of the display screen and comprising an arrangement of elongate apertures, an electron gun system arranged opposite the display screen for producing co-planar electron beams and a deflection system arranged between the electron gun system and the display screen.
The display device described above is of a type which has hitherto been conventional. The plane in which the undeflected electron beams are located is parallel to the long axis of the display screen. The orthogonal deflection fields generated by the deflection system upon energisation generally have such a (pincushion-shaped and barrel-shaped) configuration, viewed in the planes transverse to the axis of the display tube, that the display device is self-convergent.
A problem in this type of display device in its current form is the sensitivity to landing errors due to thermally or mechanically induced errors in the distance between the mask and the screen. This problem is not new, but it will become even more manifest in future HDTV systems with aspect ratios of more than 4:3, particularly 16:9, because the horizonatl deflection angles and hence the landing angles of the electron beams increase in the horizontal plane, while the diagonal deflection angle remains the same.